When driving to the end of the valley of Cogne the road ends in Lillaz at a parking lot with a fee (gps 45.595288, 7.390522). After parking your car cross the pedestrian bridge (the watercourse isn’t visible on Google maps) and go left.
From the picturesque village Lillaz (at the right side of the river going upstream) a path leads to the base of Cascate di Lillaz in a few minutes. From there you have the best view on Cascate di Lillaz. You also can walk to the right side of the waterfall where you see clearly how impressive and powerful Cascate di Lillaz is.
If you want (certainly do) you can climb up the waterfall and visit the several viewpoint. Upstream a second smaller fall shows up, but still a very beautiful waterfall! Climb further on until you cross a bridge, there a third waterfall shows up, probably the highest one of Cascate di Lillaz.
From here on you can walk back to Lillaz via the other side of the river. In total is was less then an hour walking, including stops and taking photo’s. I really love the beauty of Cascate di Lillaz and with me many tourists and locals who use the rocks at the base of the waterfall for sunbathing. I can’t imagine a better place to relax, read a good book and enjoy the sound of Cascate di Lillaz thundering down under a clear blue sky.
There are many other great waterfalls in the region of Valle d’Aosta. I think cascate del Rutor is my alltime favorite. So powerful and so beautiful. If you are near la Thuile you must visit this waterfall.
Although I visited Valle d’Aosta only once, it was one of the most beautiful region of Europe I ever visited. More information about Valle d’Aosta can be found on the tourism website, off cause
Note: I was in Italy in July/August 2014 in an extremely wet summer. The volume of waterfalls (including Cascate di Lillaz) can differ from the amount of water falling down on the pictures I took.